Magnetic (or magnetoresistive) random access memory (MRAM) is a non-volatile access memory technology that could potentially replace the dynamic random access memory (DRAM) as the standard memory for computing devices. Particularly, the use of MRAM-devices as a non-volatile RAM will eventually allow for “instant on”-systems that come to life as soon as the computer system is turned on, thus saving the amount of time needed for a conventional computer to transfer boot data from a hard disk drive to volatile DRAM during system power up.
A magnetic memory cell (also referred to as a tunneling magneto-resistive or TMR-device) includes a structure having ferromagnetic layers respectively provided with a resultant magnetic moment vector and separated by a non-magnetic layer (tunnel barrier) and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the magnetic memory cell as directions of magnetic moment vectors in the ferromagnetic layers. More specifically, the resultant magnetic moment vector of one ferromagnetic layer is magnetically fixed or pinned (conveniently also referred to as “reference layer” or “pinned layer”), while the resultant magnetic moment vector of the other ferromagnetic layer (conveniently also referred to as “free layer”) is free to be switched between two preferred directions, i.e. the same and opposite directions with respect to the fixed magnetization direction of the reference layer. The orientations of the magnetic moment vector of the free layer are also known as “parallel” and “antiparallel” states, respectively, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an antiparallel state refers to opposing magnetic alignments therebetween. Accordingly, a memory state of a magnetic memory cell is not maintained by power as in DRAMs, but rather by the direction of the magnetic moment vector of the free layer with respect to the direction of the magnetic moment vector of the reference layer.
Depending upon the magnetic states of the free layer (i.e. parallel or antiparallel states), the magnetic memory cell exhibits two different resistance values in response to a voltage applied across the magnetic tunnel junction barrier. The particular resistance of the TMR-device thus reflects the magnetization state of the free layer, wherein the resistance is “low” when the magnetization is parallel, and “high” when the magnetization is antiparallel. Accordingly, a detection of changes in resistance allows an MRAM-device to provide information stored in the magnetic memory element, that is to say to read information from the magnetic memory cell. In addition, a magnetic memory cell is written to through the application of bi- or uni-directional currents in order to magnetically align the free layer in a parallel or an antiparallel state.
An MRAM-chip typically integrates a plurality of magnetic memory cells and other circuits, such as a control circuit for magnetic memory cells, comparators for detecting states in a magnetic memory cell, input/output circuits and miscellaneous support circuitry. Conveniently, magnetic memory cells are designed to be integrated into the back end wiring structure of back-end-of-line (BEOL) CMOS processing following front-end-of-line (FEOL) CMOS processing.
To be useful in present day electronic devices, very high density arrays of magnetic memory cells are utilized in magnetic random access memory devices. In these high density arrays the magnetic cells are generally arranged in rows and columns, with individual cells being addressable for reading and writing operations by the selection of an appropriate row and column containing the desired cell. Also, conveniently orthogonal current lines are provided, one for each row and one for each column so that a selected cell is written by applying current to the appropriate row current line and the appropriate column current line.
In such a typical tri-layer arrangement of MRAM-cells, if a magnetic field having at least a vector component in the direction opposite to the magnetization direction of the free layer is applied in the direction of the easy axis (preferred direction or direction of magnetic anisotropy), then the magnetic moment vector of the free layer is reversed at a critical magnetic field value, which is also referred to as reversal magnetic field. The value of the reversal magnetic field can be determined from a minimum energy condition. Assuming that a magnetic field applied to the direction of the hard axis of magnetization is represented by Hx and a magnetic field applied to the direction of the easy axis of magnetization is represented by Hy, then a relationship Hx(2/3)+Hy(2/3)=Hc(2/3) is established, where Hc represents the anisotropic magnetic field of the free layer. Since this curve forms an astroid on the Hx-Hy-plane, it is called an astroid curve. As can be seen from the above relationship, a composite magnetic field enables the selection of a single MRAM-cell in the case where the sum of both magnetic fields at least amounts to the reversal magnetic field. Based on the above equation, a typical switching mechanism used for switching MRAM-cells is the “Stoner-Wohlfahrt”-switching scenario, well-known to those skilled in the art, in which the magnetic anisotropy of the free layer is chosen to be approximately parallel to the wafer surface.
In recent years, a new concept of magnetoresistive tunneling junction memory cells has been proposed, where the free layer is designed to be a free magnetic region including a number of ferromagnetic layers that are antiferromagnetically coupled. The number of antiferromagnetically coupled ferromagnetic layers may be appropriately chosen to increase the effective magnetic switching volume of the MRAM device. See, for instance, U.S. Pat. No. 6,531,723 to Engel et al., the disclosure of which is incorporated herein by reference in its entirety.
For switching of such magnetoresistive memory cells having a free magnetic region including antiferromagnetically coupled ferromagnetic layers, another switching scenario, the so-called “adiabatic rotational switching”, which is well-known to those skilled in the art, is envisaged. The adiabatic rotational switching mechanism is, for example, disclosed in U.S. Pat. No. 6,545,906 to Savtchenko et al., the disclosure of which is incorporated herein by reference in its entirety.
One major difference between convenient Stoner-Wohlfarth-switching and adiabatic rotational switching is given by the fact that the latter one typically uses only uni-directional currents applied to bit and word lines for switching of the resultant magnetic moment vector of the free magnetic region. More specifically, adiabatic rotational switching relies on the “spin-flop” phenomenon, which lowers the total magnetic energy in an applied magnetic field by rotating the magnetic moment vectors of the magnetic free region ferromagnetic layers.
Now reference is made to FIG. 1, where a typical stability diagram for an adiabatic rotation switchable MRAM cell is illustrated, the abscissa of which represents the bit line magnetic field HBL, while its ordinate represents the word line magnetic field HWL, which respectively arrive at the MRAM cell for its switching. Using the spin-flop phenomenon in an MRAM cell having antiferromagnetically coupled magnetic moment vectors M1 and M2 exhibited by the free magnetic region ferromagnetic layers inclined at a 45° angle to the word and bit lines, respectively, a timed switching pulse sequence of applied magnetic fields in a typical “toggling write” mode is at follows: at a time t0 neither a word line current nor a bit line current are applied resulting in a zero magnetic field H0 of both HBL and HWL. At a time t1, the word line current is increased to H1 and magnetic moment vectors M1 and M2 begin to rotate either clockwise or counter-clockwise, depending on the direction of the word line current, to align themselves nominally orthogonal to the field direction. At a time t2, the bit line current is switched on. The bit line current is chosen to flow in a certain direction so that both magnetic moment vectors M1 and M2 are further rotated in the same clockwise or counter-clockwise direction as the rotation caused by the word line magnetic field. At this time t2, both the word and bit line currents are on, resulting in magnetic field H2 with magnetic moment vectors M1 and M2 being nominally orthogonal to the net magnetic field direction, which is 45° with respect to the current lines. At a time t3, the word line current is switched off, resulting in magnetic field H3, so that magnetic moment vectors M1 and M2 are being rotated only by the bit line magnetic field. At this point in time, magnetic moment vectors M1 and M2 have generally been rotated past their hard axis instability points. Finally, at a time t4, the bit line current is switched off, again resulting in zero magnetic field H0, and magnetic moment vectors M1 and M2 will align along the preferred anisotropy axis (easy axis) in a 180° angle rotated state as compared to the initial state. Accordingly, with regard to the magnetic moment vector of the reference layer, the MRAM cell has been switched from its parallel state into its anti-parallel state, or vice versa, depending on the state switching (“toggling”) starts off with.
In order to successfully switch the MRAM cell, it is essential that the magnetic field sequence applied thereon results in a magnetic field path crossing a diagonal line and circling around a critical magnetic field value (“toggling point”) T for initiating toggle switching, since only in that case magnetic moment vectors M1 and M2 are rotated past their hard axis instability points.
Otherwise, magnetic fields applied on the MRAM cells must not arrive at another critical magnetic field value (“saturation point”) S illustrated in FIG. 1, at which both magnetic moment vectors M1 and M2 of antiferromagnetically coupled ferromagnetic layers of the free magnetic region are forced to align with the applied external magnetic field(s) in a parallel configuration.
The cut in the first quadrant in FIG. 1 leads to rectangular “astroids” with a large switching margin. In principle, the toggle field and the activation energy can be adjusted independently in this concept.
However, there are several problems to be tackled in scaling down above MRAM cells, which is one of the most important issues for low-cost and high-density MRAM devices, especially in light of modern portable equipment, such as portable computers, digital still cameras and the like. Down-scaling such MRAM cells requires smaller and smaller magnetic tunnel junctions, which is problematic, since for a given aspect ratio and free layer (or region) thickness the activation energy being dependent on the free layer (or region) volume scales down by w2, where w is the width of the magnetic cell. Otherwise, the switching fields increase roughly by 1/√{square root over (w)}. Thus, when scaling down MRAM cells, field selected switching becomes ever harder, but at the same time the magnetic cell loses its information more and more rapidly due to thermal activation. For instance, a major problem with having a small activation energy (energy barrier) is that it becomes extremely difficult to selectively switch one MRAM cell in an array, where selectability is seen to allow switching without inadvertently switching other MRAM cells. Further, a rather strong coupling of the antiferromagnetically coupled layers is required to reduce dipole coupling.
In order to solve the above problems, it has been proposed to add another ferromagnetic layer to the stack of MTJ layers to provide for a static magnetic offset field that shifts rectangular astroid in the stability diagram of FIG. 1 in such a way that the toggling point T approaches the origin of coordinates representing a zero magnetic field of both bit and word lines.
Reference is now made to FIG. 2, where the effect of an additional static magnetic offset field is illustrated. As can be seen, providing a further magnetic offset field results in a shift of rectangular astroid 1 (solid line) to rectangular astroid 2 (dotted line), and, hence, toggling point T1 is shifted to toggling point T2, which is closer to the diagram axes crossing point. Accordingly, circling around toggling point T2 for switching of the memory cell can be effected by reduced magnetic switching fields H0, H1, H2, H3 as compared to the case without application of a static magnetic offset field as shown in FIG. 1.
However, adding of a ferromagnetic offset layer, which creates a static magnetic offset field for shifting the toggling point has the drawback that such additional ferromagnetic layer reduces permanently the activation energy for switching of the MRAM cell. Aggravating this problem, this effect adds to the activation energy reduction the MRAM cell is already suffering due to its down-scaling. For that reason, inadvertent switching of MRAM cells due to thermal fluctuations is more likely to occur, which for adiabatic rotational switching is most critical in the idle state, that is to say in the state where the chip stands-by without being operated (in contrast to Stoner-Wohlfahrt-switching of convenient MRAM cells, where the most critical events are the half-select events).